The present invention concerns a procedure for controlling volumetric flow rate in air-conditioning installations, in said procedure the volumetric flow in an air-conditioning duct being controlled with the aid of a control means disposed in the air duct by opening or closing the control damper of said control means.
A major drawback of control valves and control dampers installed in air-conditioning ducts is their poor controllability. When the damper is opened, the air quantity passing through the damper begins to increase steeply, and the increase of the air flow rate levels out when the control damper reaches its other extreme position. Often, however accurate air flow control is required, and precisely that kind of control in which exactly the desired volumetric flow rate is obtained with the control variable. Control damper designs of prior art enable no such operation; the operation of the valve is highly non-linear and therefore also frequently uncontrolled.
One of the most difficult present-day problems in air flow control is the control of great air flows. Most commonly a control damper provided with turnable slats is used to control air flows.
Air-conditioning systems have to be dimensioned and controlled so that no unnecessary loss of pressure occurs in the system. Major pressure drops cause noise problems in the duct system and require bigger and more powerful blowers, and stronger ducts. These facts directly increase the building and running costs of the installation and impair its comfort in use. The same facts also give rise to special requirements regarding the components of the duct system, e.g. the control dampers. The pressure drop problem is one of the reasons why the control dampers are constructed to be congruent with the duct systems of their nominal dimensions so that they might not cause any pressure drop when fully open. For this reason the control dampers are over-dimensioned from the viewpoint of control technology. Therefore their effective operational characteristic curve is also strongly non-linear and, furthermore, their effective control range is narrow. As a result of the non-linear effective characteristic curve, the change of the control damper's control variable (from 0 to 100%) will not alter the volumetric flow in the direct proportion: in the worst case the controlled volumetric flow rate may be a multiple of that which is desired.